| dc.description.abstract | The main aim of this thesis is to prove a concept of a novel detection method for liquid organic compounds (LOCs) based on luminescent nanoparticles (NPs) that can be used in oil-field applications. Before starting an enhanced oil recovery (EOR) operation, it is important to estimate the amount of immobile oil left in the reservoir and qualify a well in terms of technical feasibility and profitability. The chemical tracers currently in use are problematic with, e.g., aquatic toxicity or low biodegradation. Therefore, new environmentally friendly detection substances of oil are needed that can replace the hazardous chemicals. Because of its high sensitivity, luminescence-based detection has already replaced radioactive tracers in many biochemical measurements. Merging luminescent molecular compounds with NPs provides opportunities for the development of new hydrocarbon detection methods.
In this work, NPs containing two luminophores were materialized by two different synthesis approaches. The first approach involved SiO2 NPs and the second core-shell composites. A modified Stöber method was used to obtain the SiO2 NPs with incorporated EuIII:EDTA complex and the water-insoluble organic dye BODIPY. The EuIII:EDTA complex had a strong influence on the resulting SiO2 nanoparticle size and distribution. Transmission electron microscopy (TEM), dynamic light scattering (DLS), zeta potential measurements, and photoluminescence (PL) measurements were used to find an optimal amount of the EuIII:EDTA solution and the molar ratio of the complex. The 1:4 molar ratio of the EuIII:EDTA was found to be optimal, giving NPs with a diameter of ~ 100 nm (suitable for transport through the porous oil reservoir structure) and detectable EuIII PL emission intensity in a dilute aqueous dispersion (> 0.1 wt%). The monodisperse NPs had a high absolute zeta potential (-44 mV) and were stable against agglomeration in an aqueous dispersion. Aggregates of the EuIII:EDTA complex were found to be in the core of the SiO2 NPs and were therefore shielded from any contact with the external environment. The shielding effect of silica prohibited the EuIII PL quenching effect of H2O and contributed to a long EuIII PL lifetime (~ 0.6 ms).
The low-temperature conditions for the EuIII:EDTA complex incorporation allowed simultaneous incorporation of the second luminophore, BODIPY, into the SiO2 NPs. The PL intensity of the dye in the NPs became higher with increasing BODIPY input concentration within the concentration range examined. In this range, self-quenching of the BODIPY PL was not observed, suggesting well distanced and homogenously distributed dye molecules throughout the spherical SiO2 NP volume.
The ability of the doubly luminescent SiO2 NPs to detect LOCs via ratiometric PL intensity was demonstrated by using toluene and 1-octanol as model compounds of crude oil. The PL emission intensity of the BODIPY in the NPs decreased after an aqueous dispersion of the NPs had been in contact with toluene or 1-octanol because of the transfer of the dye molecules from the NPs to the LOCs. The EuIII PL emission intensity was unaltered by the changing environment. The modulated intensity ratio of the two PL emissions permitted a quantitative determination of the BODIPY loss. The amount of the BODIPY released from the NPs was approximately the same in toluene and 1-octanol, with a maximum of ~ 60 % after an induced contact between the two phases for 24 h. Similar release kinetics observed for both solvents indicated that the SiO2 NPs' encapsulation governed the process of BODIPY release, i.e., mostly mechanically promoted desorption.
The doubly luminescent SiO2 NPs were also synthesized with varying amounts of the Pluronic® copolymers L-121 and P-123. However, the copolymers neither increased the BODIPY loading into the SiO2 NPs nor contributed to a higher dye release upon contact with the LOCs. Thermogravimetric analysis of the NP samples showed relatively small mass losses, implicating small fractions of the Pluronics® incorporated. On the other hand, the effect of the copolymers on the NP surface properties was significant. The aqueous dispersions of the Pluronic® containing NPs had smaller absolute zeta potential than those without the copolymers and were less stable in terms of agglomeration because of lower electrostatic repulsion between the NPs.
Toxicity tests were conducted for SiO2 NPs containing the EuIII:EDTA complex, the BODIPY, and the Pluronics®. The cytotoxic potential of the NPs was found to be very low for human lung cell and fish cell lines. Even in the most sensitive colony forming efficiency cytotoxicity assay, no significant effects were observed at concentrations as high as 380 μg/mL.
The ability of the doubly luminescent SiO2 NPs to detect LOCs, along with the confirmation that they are non-toxic, demonstrates their potential to replace the hazardous chemicals used to estimate the amount of immobile oil left in the reservoir and improve the planning of an EOR operation. Moreover, the ratiometric PL detection technique is anticipated to be of benefit in other fields, such as biotechnology and medical diagnostics, where a reliable and safe detection of a liquid organic phase is needed.
Next, core-shell composites were synthesized as the second approach to prepare NPs containing two luminophores. The core, a phosphor with stable EuIII PL, i.e., EuIII-doped NaLa(WO4)(MoO4), was synthesized first by a wet chemical synthesis route. Toxicity tests of the phosphor on human lung cell and fish cell lines showed no significant cytotoxic effects at concentrations up to 160 μg/mL. The EuIII-doped NaLa(WO4)(MoO4) was subsequently coated in two different methods. The coating with BODIPY, Pluronic®, and the coupling agent (3-aminopropyl)triethoxysilane (APTES) resulted in a 5 nm thick shell. However, this coating layer was observed to detach from the core upon contact with 1-octanol, which demonstrated a need for a stronger and more robust shell to host the dye molecules. Silica coating of the EuIII-doped NaLa(WO4)(MoO4) resulted in a 30 nm thick shell, providing a robust host layer for BODIPY as the second organic luminophore. Incorporating the BODIPY into the silica shell was not pursued, although this core-shell composite might be promising for further development into doubly luminescent particles, which can be used to detect a LOC phase.
In the present work, a third particle type consisting of silica and luminescent carbon dots (CDs) was also prepared. Hydrothermal treatment of microporous silica particles in the presence of urea and citric acid at low temperature (160 ºC) resulted in luminescent, microporous, and hollow nanocomposites (silica-CDs) with a surface area of 12 m2/g. Their luminescence emission at 445 nm was remarkably stable in an aqueous dispersion under a wide pH range (3-12) and in the dried state. The toxicity of the silica-CDs was in general low on human lung cell and fish cell lines at exposures up to 10 μg/cm2, which demonstrated their biocompatibility and potential for applications in, e.g., bioanalytical assays or as drug carriers where stable luminescence under varying conditions is needed. | en_US |
